1. Field of the Invention
The present invention relates to plastic compositions which are electrically conductive. Indeed, the present invention is particularly concerned with the preparation of electrically conducting nonporous, thin sheet elements from polyolefins useful especially in the construction of conductive plates for electrochemical cells.
2. Prior Art
Many attempts have been made to make conductive or semiconductive materials from polymeric plastics loaded with conductive solids such as carbon black, graphite or finely divided metals. All manners of thermoplastic and thermosetting resins have been proposed including melamine, phenol-aldehyde, and more commonly polyolefins such as polyethylene and graft copolymers thereof. See, for example, U.S. Pat. No. 3,591,526 and U.S. Pat. No. 3,673,121.
In general, compositions of relatively low resistance can be prepared by dry mixing a finely divided thermoplastic polymer and conductive filler and molding the mixtures under heat and pressure. Such molded products are normally porous and nonhomogeneous in structure, and, accordingly, are not suitable for certain sophisticated applications which require thin impermeable conductors of highly uniform composition. An example of such a sophisticated application is a bipolar plate for a fuel cell or battery. In this regard, see, for example, U.S. Pat. No. 3,814,631 and U.S. Pat. No. 3,530,003.
A far higher degree of homogeneity than is obtained by a dry blending process can be obtained by the use of known mixing devices such as a Banbury mixer or roll mill. French Pat. No. 1,305,140 describes the preparation in a Banbury mixer of a number of blends of carbon black or graphite in a crystalline polypropylene with or without an amorphous copolymer plasticizer. The resistivities of these blends are all of an order of a number of megaohms-centimeter and they are described as suitable for use as thermistors and semiconductors rather than for use in highly conductive applications. Moreover, such carbon-plastic blends are not capable of being easily formed, for example, by extrusion into thin nonporous sheet-like structures, particularly thin sheet elements having thicknesses of, for example, from about 150 microns to about 500 microns.
As should be readily appreciated, when working with then sheet elements having thicknesses, for example, in the range from about 150 microns to about 500 microns, the physical strength of such material becomes an important mechanical property. Thus, while it was known in the past to add carbon to polymers to increase their electrical conductivity, frequently the blended material made in a Banbury mixer will be brittle. Indeed, many such materials will snap completely in pieces when an attempt is made to bend them at a 90.degree. angle.
In addition to having the requisite flex strength for sophisticated applications such as that of the bipolar plate for a fuel cell or battery, it is highly desirable that the requisite composition not shrink upon cooling from a melted condition and be capable of being thermally bonded to other plastic materials, for example, such as a frame for a bipolar plate or to a metal surface such as a current collector. Many of the foregoing properties would appear almost to be impossible to achieve in a single composition. For example, it is known that plastics can be reinforced with fibers such as glass. An example of such compositions can be found, for example, in U.S. Pat. No. 3,853,808 and U.S. Pat. No. 3,819,568. As will be readily appreciated, the addition of glass, however, tends to decrease the electrical conductivity of compositions containing such material.
In U.S. Pat. No. 4,002,595, it is reported that the addition of about 1% of finely divided silica to polyethylene filled with carbon black increased the conductivity only slightly and that 3-5% additions of such finely divided silica decreased the conductivity. The patent further indicates, however, that addition of various nonconductive fillers in natural rubber, for example, plays no part in determining the resistivity and that the latter is determined only by the relative quantity of carbon black and filled compound.
Thus, it should be readily apparent that the performance of compositions described in the literature leaves much to be desired. For reasons cited herein, it has been difficult to prepare compositions which are exceedingly homogeneous, which have great flex strength, are highly electrically conducting, are injection moldable and are impermeable, for example, to bromine when used as bipolar plates in a metal bromide battery.